I've reflected on what 'unacceptable behaviour' may be, in response to various posts. I agree totally that abuse of children by parents is never acceptable behaviour.
Going back to the OP, violence from children to parents is unacceptable behaviour. The age of the child is significant. A toddler may smack out despite never experiencing that behaviour themselves. Its part of learning to manage emotions and parents are key in that. A child of 7 causing their mother to fall to the ground, by 'drop kicking' her knees is unacceptable. That mother needs help with her parenting skills, that should be led by a proper assessment of the family including 1-1 with the child.
Move on to an adolescent who is using substances, behaving abusively towards their parents and siblings. Unacceptable behaviour, help needed for all concerned.
Adult children threatening their parents, stealing from them etc, unacceptable behaviour. The majority of parents in those situations try hard to encourage their child to get appropriate help. The parents seek out advice on how to help their child, fearful that if they stop treading egg shells, their child will estrange from them.